Recently, a flat panel display (FPD) is being more and more reduced in the thickness of the display panel. Reduction of the panel thickness is aggressively proceeding particularly in a display panel for portable terminal equipment such as cellular phone and PDA so as to meet the requirement for a lightweight device.
As a technique for reducing the thickness of the display panel, a method of laminating together a CF substrate and a TFT array substrate in a color liquid crystal display device and then, thinning the outer surface of the glass substrate by a chemical or physical method (e.g., chemical etching etc.) is performed.
For example, a finished panel having a thickness of 0.7 mm which is a standard thickness of generally produced sheet glass for FPD or having a slightly small thickness of 0.5 to 0.6 mm is sometimes subjected to etching so as to reduce the thickness to 0.2 to 0.3 mm.
Conventionally, when a glass substrate has some degrees of thickness, there is no problem with the strength thereof, because the glass substrate itself originally has high strength, but the above-described reduction in the thickness of the glass substrate brings about a new problem that the strength of the finally formed glass substrate decreases.
In a color liquid crystal display device, the color filter substrate or the like used therefor is provided with a black matrix (BM) so as to enhance the display quality including display contrast of the image. This BM shades the periphery of the display portion of each color pixel to prevent colors from decoloration of each color of three primary colors R, G and B adjacent to each other of a color filter and is generally used to improve the contrast of color display and increase the display quality.
Important characteristics as BM include optical characteristics, specifically, high light shielding ratio and low reflectivity. In order to prevent color mixing due to colors from decoloration of respective colors, unnecessary light from a light source needs to be sufficiently shielded. Also, in order to prevent external light from reflecting on the display side (observer side) and disallow reflected light to reduce the contrast of the display image, the reflectivity needs to be low.
A highly light-shielding metal film, particularly a chromium film, has been conventionally used as a light-shielding film for color liquid crystal display devices. The chromium film has high light-shielding property but its visible light reflectivity is about 50% and for more increasing the contrast, the visible light reflectivity needs to be more lowered. Therefore, a method of laminating a chromium oxide film on the chromium film and reducing the visible light reflectivity by using light interference has been proposed.
In order to meet the requirement for a light-shielding film with a lower visible light reflectivity, the applicant of the present invention had proposed the followings in Patent Document 1. (1) A light-shielding film for liquid crystal display devices, which is provided on a transparent substrate, the light-shielding film having a layer structure consisting of a chromium oxide film and a chromium nitride film from the transparent substrate side; (2) the light-shielding film for liquid crystal display devices as described in (1) above, which has a metal chromium film on the chromium nitride film to the transparent substrate; and (3) the light-shielding film for liquid crystal display devices as described in (1) or (2) above, wherein a chromium oxynitride film is used in stead of the chromium oxide film.
Also, the applicant of the present invention had proposed the followings in Patent Document 2.
(1) A light-shielding film for liquid crystal display devices, which is provided on a transparent substrate, the light-shielding film having a layer structure consisting of a chromium oxynitride film and a chromium film from the transparent substrate side, wherein the composition of the chromium oxynitride film, as expressed by Cr1-x-yOxNy in terms of the element concentration, is in the following ranges:0.30≦x≦0.55, and0.03≦y≦0.20.
(2) A liquid crystal display device which uses, as one substrate for interposing a liquid crystal layer therebetween, a substrate comprising a transparent substrate, a color filter film formed in the portion corresponding to a pixel on the transparent substrate, the light-shielding film for liquid crystal display devices described in (1) above, which is formed in the portion corresponding to an area between pixels, and a transparent electrode film formed thereon.
These light-shielding films for liquid crystal display devices can greatly decrease the reflectivity over the entire visible region as well as in the vicinity of a wavelength of 555 nm where the spectral luminous efficiency becomes maximum, compared with a conventional light-shielding film composed of a chromium oxide film and a chromium film. Accordingly, such a light-shielding film for liquid crystal display devices makes it possible to obtain a light-shielding film having a sufficiently low reflectivity in the visible region, specifically, a light-shielding film having a luminous reflectivity (Y) of about 5%, which cannot be realized in a conventional light-shielding film composed of a chromium oxide film and a chromium film by any thickness adjustment.
In this way, the development of conventional light-shielding film-attached glass substrates mainly puts the focus on enhancement of optical characteristics, and the relationship between the light-shielding film and the strength of the liquid crystal display device has not been discussed.
However, the thickness reduction of a liquid crystal display device for portable terminal equipment such as cellular phone and PDA is accompanied by a requirement to enhance the breaking strength of a liquid crystal display device. In turn, the light-shielding film-attached glass substrate used for the production of a liquid crystal display device is also required to improve in the breaking strength.
For example, the light-shielding films for liquid crystal display devices described in Patent Documents 1 and 2 are excellent from the aspect of low reflectivity in the visible region but are not necessarily sufficient in terms of breaking strength of the light-shielding film-attached glass substrate.
Conventionally, the strength of the light shielding film-attached glass substrate is thought to dominant in the strength of the glass substrate itself accounting for a majority of the volume, and it has not been considered that the light-shielding film very thinly formed on the glass substrate surface has a great effect.
[Patent Document 1] JP-A-8-36171
[Patent Document 2] JP-A-8-29768